I was also thinking about buying two 7850s or 7870s - it's the same price as for one 7970, but the problem is the future expanding. I can always buy another 7970 for crossfire in 1-2 years.
But from the other side, I have now only half of the money needed for 7970 and it would be great to have 7870 right now, and just buy next one soon.
I'm patient and I will wait for 7970 and I hope it will be a little cheaper for Christmas.
Any more thoughts?